Hello
I was wondering, as bare metal programming is about changing the kernel.img file of the SD card, replacing it on the RPi, testing, removing it, changing the kernel.img..., if there are no risks for the RPi SD card reader to get worn (even if I remove and replace the card carefully)?
Thank you
Cathy L.

If you are worrying about it, you can use this: https://github.com/mrvn/raspbootin
It will use a fix kernel.img on the SD card which will load your real kernel.img over serial. No need to remove the SD card at all.

I agree, go with a bootloader. the original pi something about that full sized slot would crack a corner of my sd card adapters, ran through a number of them (before having a solid bootloader). later adafruit made a pcb flush mount micro sd adapter that would slide in and that problem got solved.